The present disclosure relates to systems and methods that process signals over a cable transmission network.
Although Cable Television (CATV) networks originally delivered content to subscribers over large distances using an exclusively RF transmission system, modern CATV transmission systems have replaced much of the RF transmission path with a more effective optical network, creating a hybrid transmission system where cable content originates and terminates as RF signals over coaxial cables, but is converted to optical signals for transmission over the bulk of the intervening distance between the content provider and the subscriber. Specifically, CATV networks include a head end at the content provider for receiving RF signals representing many channels of content. The head end receives the respective RF content signals, multiplexes them using an RF combining network, converts the combined RF signal to an optical signal (typically by using the RF signal to modulate a laser) and outputs the optical signal to a fiber-optic network that communicates the signal to one or more nodes, each proximate a group of subscribers. The node then reverses the conversion process by de-multiplexing the received optical signal and converting it back to an RF signal so that it can be received by viewers.
Cable television (CATV) networks have continuously evolved since first being deployed as relatively simple systems that delivered video channels one-way from a content provider. Early systems included transmitters that assigned a number of CATV channels to separate frequency bands, each of approximately 6 MHz. Subsequent advancements permitted limited return communication from the subscribers back to the content provider either through a dedicated, small low-frequency signal propagated onto the coaxial network. Modern CATV networks, however, not only provide for a much greater number of channels of content, but also provide data services (such as Internet access) that require much greater bandwidth to be assigned for both forward and return paths. In the specification, the drawings, and the claims, the terms “forward path” and “downstream” may be interchangeably used to refer to a path from a head end to a node, a node to an end-user, or a head end to an end user. Conversely, the terms “return path” “reverse path” and “upstream” may be interchangeably used to refer to a path from an end user to a node, a node to a head end, or an end user to a head end.
Recent improvements in CATV architectures that provide further improvements in delivery of content include Fiber-to-the Premises (FTTP) architectures that replace the coaxial network between a node and a subscriber's home with a fiber-optic network. Such architectures are also called Radio Frequency over Glass (RFoG) architectures. A key benefit of RFoG is that it provides for faster connection speeds and more bandwidth than current coaxial transmission paths are capable of delivering. For example, a single copper coaxial twisted pair conductor can carry six simultaneous phone calls, while a single fiber pair can carry more than 2.5 million phone calls simultaneously. Furthermore, coaxial cable, depending on the type/size/conductor, may have tens of dBs of losses per hundreds of feet (and the higher the RF frequency desired, the higher the coaxial cable losses). In HFC networks these losses require placement of in-line RF amplifiers. Conversely, optical FTTP has fewer losses and no need for in-line amplifiers. FTTP also allows consumers to bundle their communications services to receive telephone, video, audio, television, any other digital data products or services simultaneously.
In the upstream path, respective optical signals from groups of customer are combined together by one or more combining networks. These combining networks typically include a number of photodiodes that receive upstream optical signals and convert them to electrical signals where they may be combined, amplified, multiplexed, and converted back to an optical signal for further upstream transmission to the next point in the network. Oftentimes one or more photodiodes in such combining networks, or the laser that retransmits the signals in the upstream direction, will fail, requiring repair, during which the combining network is non-operational and service is interrupted. What is desired, therefore, is an improved system that ameliorates this service interruption.